1. Field of the Invention
The present invention relates to a radiographic system, an automatic exposure control method of a radiographic system, and a radiological image detector.
2. Description of the Related Art
In the medical fields, radiographic systems using radiation such as X-rays are known. A radiographic system includes an X-ray generator generating X-rays and an X-ray imaging apparatus receiving X-rays and capturing an X-ray image. The X-ray generator includes an X-ray source irradiating an object with X-rays, a source controller controlling driving of the X-ray source, and an irradiation switch used to input an X-ray irradiation start instruction. The X-ray imaging apparatus includes an X-ray image detector receiving X-rays passing through an object and detecting an X-ray image and a console controlling driving of the X-ray image detector and performing various image processes on the X-ray image.
In the field of recent radiographic systems, X-ray image detectors using a flat panel detector (FPD) as a detection panel instead of an X-ray film or an imaging plate (IP) have been widespread. In the FPD, pixels accumulating signal charges corresponding to an amount of incident X-rays are arranged in a matrix shape. The FPD accumulates signal charges in the respective pixels and converts the accumulated signal charges into a voltage signal through the use of a signal processing circuit, whereby an X-ray image representing image information of the object is detected and is output as digital image data.
An electronic cassette (portable X-ray image detector) in which the FPD is built in a housing of a rectangular parallelepiped shape is also put into practice. Unlike a type fixed to a radiography platform and not detached therefrom, the electronic cassette is detachably attached to an existing radiography platform for a film cassette or an IP cassette or a dedicated radiography platform for use, or is placed on a bed in order to radiograph a site which it is difficult to radiograph by the use of the fixed type or is made for an object to carry for use. The electronic cassette may be used in the outside of a hospital, in which there is no radiography platform, in order to radiograph an aged person under home remedy or an emergency patient under an accident or a disaster.
The radiographic system is provided with a sensor such as an ion chamber (ionization chamber) detecting an X-ray dose passing through an object and performs an automatic exposure control (AEC) stopping irradiation with X-rays from an X-ray source when the integrated value of the X-ray dose detected with the sensor reaches a predetermined threshold.
JP2004-223157A discloses that the arrangement of AEC detection elements is devised so that the positions of the AEC detection elements (AEC detection areas, detection fields) are suitable even when an electronic cassette is used in any of vertical setting and horizontal setting, and the AEC detection elements to be used are determined depending on the vertical setting or the horizontal setting. JP2002-000590A discloses that a pixel area (a detection field) of an FPD used as an AEC sensor is selected depending on detection sites of interest (such as abdomen and chest) of an object, the added value or the average value of signals of the pixels in the selected area is calculated, and an X-ray blocking signal (irradiation stop signal) is generated on the basis of the comparison result of the added value or the average value with a set concentration value (irradiation-stop threshold value).